1. Field of the Invention
Embodiments of the present invention generally relate to an integrated circuit in a hard disk drive. Specifically, the invention relates to circuits for driving a heating element to aid in writing data to high coercivity media.
2. Description of the Related Art
Perpendicular magnetic recording, where the recorded bits are stored in a planar recording layer in a generally perpendicular or out-of-plane orientation (rather than parallel to the surface of the recording layer), is a one path toward ultra-high recording densities in magnetic recording systems, such as hard disk drives. The perpendicular magnetic recording layer is typically a continuous layer on the disk substrate, like in conventional magnetic recording disk drives. However, magnetic recording disk drives with patterned perpendicular magnetic recording layers increase data density by recording bits in a perpendicular orientation. In patterned media, the perpendicular magnetic recording layer on the disk is organized into small isolated data islands arranged in concentric data tracks. To produce the magnetic isolation of the patterned data islands, the magnetic moment of the spaces or regions between the data islands is not present or substantially reduced to render these regions essentially nonmagnetic. Alternatively, the media may be fabricated so that there is no magnetic material in the regions between the data islands.
A problem associated with continuous perpendicular magnetic recording media is the thermal instability of the recorded magnetization patterns. In continuous perpendicular magnetic recording layers, the magnetic material (or media) for the recording layer on the disk is chosen to have sufficient coercivity such that the magnetized data bits are correctly written and retain their magnetization state until written over by new data bits. As the areal data density (the number of bits that can be recorded on a unit surface area of the disk) increases, the magnetic grains that make up the data bits can be so small that they can be demagnetized simply from thermal instability or agitation within the magnetized bit (the so-called “superparamagnetic” effect). To avoid thermal instabilities of the stored magnetization, media with high magneto-crystalline anisotropy (KU) may be required. However, increasing KU in recording media also increases the switching field, H0, which is proportional to the ratio KU/MS, where MS is the saturation magnetization (the magnetic moment per unit volume). The switching field H0 is the field required to reverse the magnetization direction when the magnetic media is subjected to short time intervals. For modern hard disk drives, this time interval is around 1 ns.
One approach to addressing the problem of providing a strong enough switching field H0 for high coercivity media is thermally-assisted recording (TAR) using a magnetic recording disk like that described in U.S. Pat. No. 6,834,026 B2, assigned to the same assignee as this application. This disk has a bilayer medium of a high-coercivity, high-anisotropy ferromagnetic material like FePt as the storage or recording layer and a material like FeRh or Fe(RhX) (where X is Ir, Pt, Ru, Re or Os) as a “transition” layer that exhibits a transition or switch from antiferromagnetic to ferromagnetic (AF-F) at a transition temperature less than the Curie temperature of the high-coercivity, high-anisotropy material of the recording layer. The recording layer and the transition layer are ferromagnetically exchange-coupled when the transition layer is in its ferromagnetic state. To write data, the bilayer medium is heated above the transition temperature of the transition layer with a separate heat source, such as a laser or electrically resistive heater. When the transition layer becomes ferromagnetic, the total magnetization of the bilayer is increased, and consequently the switching field required to reverse a magnetized bit is decreased without lowering the anisotropy of the recording layer. The magnetic bit pattern is recorded in both the recording layer and the transition layer. When the media is cooled to below the transition temperature of the transition layer, the transition layer becomes antiferromagnetic and the bit pattern remains in the high-anisotropy recording layer.
Generally, a laser may be focused onto a spot of the magnetic disk (i.e., a single bit) to heat up the spot and lower the coercivity of the magnetic material. A write head then projects the desired magnetic field through the heated spot. The magnetic material of the spot then aligns with the magnetic field. As the spot cools, the coercivity increases and stabilizes the magnetic field of the high-anisotropy layer. Thus, a read pole is able to pass over the spot, detect the magnetic field, and interpret the bit pattern.
Ideally, the laser is focused only on the bit whose magnetic orientation will be changed by the write head. Heating up surrounding bits lower their coercivity and increases the risk that their orientations will be changed by the write head. Unfortunately, light's diffraction limit generally prevents lenses from focusing down a beam spot to less than half of the light's wavelength. Given the wavelength of optical lasers, lenses can focus the light to around 200 nm. If a 1 Tb density is to be achieved, the spot size of the laser should closely follow the width of a bit—i.e., tens of nanometers. Recently, different mirrors or waveguides may be used to focus light down to a quarter of its wavelength. However, this still does not produce a beam spot that focuses solely on a single bit of a magnetic disk.
What is needed is an apparatus that minimizes the effect of a laser's beam spot on surrounding bit patterns.